Optical levitation is a technique for supporting transparent particles by the forces of radiation pressure from laser beams ("The Pressure of Laser Light", by A. Ashkin, Scientific American, 226, 63 (1972)). When optical levitation is done in air, the damping forces provided by the viscosity of the air control perturbations and hold the particles steady. When particles are suspended in high vacuum, the viscosity becomes negligible, and fluctuations in beam power, vibrations of the apparatus, etc. will introduce perturbations in the particle position that will die out only after a long time. This effect is discussed in a paper by the present inventors ("Optical Levitation in High Vacuum", A. Ashkin and J. M. Dziedzic, Applied Physics Letters, 28, 6, Mar. 15, 1976, pp. 333-335), which discusses the decay time of perturbations as a function of residual pressure. It would clearly be highly desirable to provide some means of damping, so the particle returns quickly to its equilibrium position.
In a similar art, that of magnetic levitation of small steel spheres, perturbations are damped by use of an external damping mechanism, comprising an on-axis steel rod mounted in a pot of viscous fluid ("Magnetic Suspension for Small Rotors", by J. W. Beams, Review of Scientific Instruments, 21, No. 2, (1950), FIG. 1). This particular arrangement is obviously inapplicable to optical levitation, not only because it depends on magnetism, but also because the use of external elements requires that the particle suspended be large and the excursion from the stable position also be large in order to produce an appreciable effect on an external damping apparatus.